This invention relates to a spark plug and a method of producing the same, and more particularly to a spark plug whose spark voltage can be kept to a low level for a long period of time and a method of producing such a spark plug.
Recently, internal combustion engines used, for example, in automobiles, have been required to have an energy-saving design to achieve a low fuel consumption, and therefore a high-compression design of the internal combustion engine, as well as a lean burn design using a lean air-fuel mixture, has been adopted.
As a result of such high-compression design and such lean burn design of the internal combustion engine, the spark voltage of a spark plug needs to be high so as to ignite the lean mixture. Meanwhile, considering recent automobile circumstances, despite a limited capacity of a battery of the automobiles, the electric power consumption has been increased. For this reason, a demand for low spark voltage of the spark plug has now been required.
To meet such requirement, there have been proposed spark plugs which can produce an electrical discharge with a spark voltage much lower than before. For example, Japanese Patent Examined Publication No. 59-33949 and Japanese Utility Model Unexamined Publication No. 53-64925 disclose a spark plug of the type in which a groove (or recess-projection portion) is formed at or near a discharge surface of a central electrode. As a result of thus forming the recess-projection portion at the discharge surface of the electrode, edges are formed at the discharge surface, and these edges cause a spark to easily leap, thereby enabling the low spark voltage.
In the discharge surface construction of the above electrode, in order to maintain the low spark voltage effect for a long period of time, it is necessary that the recess-projection portion should remain to the last even when the consumption of the electrode proceeds. Therefore, in view of the lifetime of the spark plug (that is, the amount of exhaust of the discharge portion due to the electrode consumption) which is another important factor in the design of the spark plug, the groove or the recess-projection portion must have sufficient initial dimensions or depth. However, the discharge surface of the above configuration is small in cross-sectional area because of the provision of the groove or the recess-projection portion, and is subjected to a very severe consumption, and therefore the depth of the groove must be very large in order to maintain the low spark voltage effect for a long period of time.
The low spark voltage effect obtained with the conventional discharge surface of the above configuration will now be described in detail with reference to FIG. 9.
FIGS. 9(a) to (d) show the shape of the discharge portion 90 of the above electrode which varies with the spark discharge time.
As shown in FIG. 9(a), a discharge gap is formed by a discharge surface of an earth electrode (not shown) and a discharge surface 90a defined by a cross-shaped groove of the discharge portion 90 of the central electrode, and therefore the discharge can be effected more easily at edges 90b, provided by the cross-shaped groove, as compared with a flat discharge surface having no recess and no projection. Therefore, at an initial stage of the discharge, the spark voltage can be made sufficiently low. Thereafter, as the discharge time becomes long with the discharge portion 90 consumed, the edges 90b of the discharge surface 90a of the discharge portion 90 become rounded, as shown in FIGS. 9(b) and 9(c). As a result, the spark discharge voltage increases, and sector-shaped pillar portions 95 separated from one another by the cross-shaped groove become short as the consumption of the discharge portion 90 proceeds.
When the discharge further continues, the pillar portions 95 of the discharge portion 90 become sufficiently short to finally reach a flat portion 90c of the discharge portion 90, as shown in FIG. 9(d). Thus, the edges 90b at which the discharge can be effected easily are eliminated, so that the spark voltage reduction effect is lost.
Therefore, with the configuration of the conventional discharge portion 90, the lifetime of the discharge portion 90 can be prolonged only by increasing the height of the sector-shaped pillar portions 95, in which case the flat portion 90c of the discharge portion 90 becomes useless, and besides there has been encountered a problem that it is very difficult to form a deep cross-shaped in the discharge surface.
More particularly, in the conventional spark plug in which the recess-projection portion is formed at the discharge end of the electrode in order to achieve the low spark voltage effect, it is difficult to maintain the low spark voltage effect for a long period of time, and the flat portion of the discharge portion is useless. Furthermore, the machining of the discharge portion to form the groove is difficult.
Further, the following problems have also been encountered where the discharge portion of the above configuration is formed by a separate discharge member.
Namely, for fixedly securing the discharge member of the above configuration to the central electrode, a flat portion of the discharge member has heretofore been welded to the end face of the central electrode by resistance welding. However, in the case where the recess-projection portion is formed at the discharge member, the cross-sectional area of the discharge member is smaller at that portion than at the flat portion, and therefore the discharge member can be easily buckled at its recess-projection portion by the heat and pressure applied during the resistance welding. Therefore, conventionally, after the discharge member is joined to the central electrode by resistance welding, the discharge portion must be cut or worked by a sheet-like cutter or the like to form the groove or the recess-projection portion.
However, in such a production method, a considerable amount of the material is cut or removed from the discharge member in order to form the groove or the recess-projection portion. Thus, much time and labor are needed, and therefore this method is not economical, and the production efficiency is low.